King of all tides

The highest tides occur with the coming of the full and new moons, and many believe these natural events coincide with a rise in madness, hence the term 'lunacy'. There are two areas in Australia that experience particularly large king tides - far north Queensland and north-west Western Australia.

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In days gone by, coastal people used the regular cycle of tides to mark the passage of time instead of the clock. The tide is frequently represented in literature as something unstoppable, which cannot be harnessed by humans. As the Scottish poet Robert Burns observed: "Nae man can tether time or tide". When our circumstances change, the tide is said to be turning, and we often speak of moving 'with' or 'against' the tide.

King tides — exceptionally high tides — have a particular fascination for both science and society. They conjour up images of whole towns being drowned under water, then the sea emptying and leaving fish flopping and gasping on the sand.

There are two areas in Australia that experience particularly large king tides — far north Queensland and north-west Western Australia. In Queensland, these tides occur from December to February, and June to August. They frequently invade coastal towns, and can flood the main streets of big cities like Cairns. Tides of up to 4.78m have been recorded at Karumba at the bottom of Cape York Penninsula, while the town of Broadsound, between Rockhampton and Mackay has an 8m tide. But these are ripples compared to the biggest tides of all in Australia.

The king of all Australian tides occurs near the town of Derby in King Sound, in north-west Australia, at the end of March and again at the end of April each year. Derby's tides can reach up to 11.8 m, and are the second biggest tides in the world (the largest, clocked at 15 m, occur in the Bay of Fundy, Nova Scotia). At the other end of the scale, the tides of the Mediterranean — the smallest in the world — peak at a weeny 2 to 3cm.

Fact file:

When: North west Australia - March and April Queensland - December to Feb and June to August

Where: Australia's biggest tides occur off the north-west coast of Western Australia (around Derby in the King Sound) and in far north Queensland.

Other info: The highest of all Australian tides and the second highest tide in the world happens near Derby, WA, in late March and again in late April. The tides peak at 11.8 metres and drop to the other extreme of about 1.5m at low tide.

The smallest tides in the world occur in the Mediterranean, peaking at just 2-3cm.

Living with a King Tide

Derby is proud of its strange tides, and holds special events to celebrate, including King Tide Day and the Boab Festival. A highlight of the festival is the annual Mud Football competition.

Derby local, John Silver, works as Regional Coastal Facilitator for the 9000km of coastline of the Pilbara and Kimberly Region. He says that when the tide goes out, it looks like the whole ocean is empty: "It's like in an old fable or kid's story. The intertidal zone (the area between low and high tides) can be tens of kilometres wide — compared to just a few metres on the east coast of Australia."

When the tide comes in, it does so at a walking pace, Silver says. "One minute, there will be no water under the jetty, the next minute it will be full. I remember as a kid when we would get a cyclone and a storm surge, and parts of the town would be underwater." Others tell stories of people getting caught out on local beaches at low tide. The tide comes in so fast, they have to make a run for it to beat the water back to shore.

The fast-moving currents create some weird, almost supernatural effects around the area, such as horizontal waterfalls and whirlpool clusters.

Why Derby?

Normal high tides are caused by a bulge in the water covering the surface of the earth. High tides occur on both sides of the earth at the same time. The water on the side closest to the moon bulges out because it is being sucked by the gravitational force of the moon. The water on the side furthest away from the moon also bulges out, because it is being pushed by forces including the centrifugal force of the earth.

But how come the tides are so VERY big at King Sound near Derby? Oceanographers call these tides "macro" tides, rather than king tides. Macro tides are much larger than the normal high tides caused by the action of the moon, because they are also influenced by the shape of the local coast. King Sound, where Derby is situated, is an extension of the wide, shallow north-west continental shelf. This has a lot to do with why this area gets such huge tides.

When the water comes in from deeper areas, it is constricted and squeezed by the shallow continental shelf. Because the volume of water is being constrained in this shallow region, the currents increase to try and move the water onto the shelf. The tide is therefore amplified as it moves shoreward. Tides at Broome are very high, at 9m. But the tides are even higher in King Sound, because this area is shaped like a huge shallow channel. Here, the water is constrained on both sides and forced up the Sound, causing these massive tides and creating the King of all Australian tides at Derby.